1. Field of the Invention
The present invention relates to a lithium ion rechargeable cell, and more specifically, to a cell case of a lithium ion rechargeable cell.
2. Description of the Related Art
In general, a lithium ion rechargeable cell is modularized. One module is constructed in the following manner. First, an electrode group, in which plate-like positive and negative electrodes and a plate-like separator are alternately superposed in layers, is packed in a cell case and sealed together with an electrolyte. The result is a cell unit. Several cell units are aligned and packed in a module case to produce a module.
A cell case and a module case have been generally fabricated with an undeformable rigid case that is made of, for example, resin or metal.
Therefore the capacity of the module case made with a rigid case is previously determined, so that the total cubic volume of the cell units contained in the module case is also previously determined.
If the cell case is made with a rigid case similarly to the module case as in the past, the relatively thick walls of the cell case limit the number of electrode plates that can be packed in the cell case. The capacity of the cell case is accordingly limited, too.
In order to enhance the workability of packing an electrode group in the cell case, an extra space is needed in the cell case. If the cell case is fabricated with an undeformable rigid case, it is difficult to make an efficient extra space, and the containing capacity is also limited.
Since the cell case fabricated with a rigid case is limited in capacity, it is difficult to achieve a compact and high-capacity lithium-ion rechargeable cell module.
If the capacity of each cell case is increased, the total cubic volume of wall thickness of the cell cases is reduced as a whole. On the other hand, as a result of increase of the cubic volume of each cell case, the number of cell cases that can be packed in a module case is decreased. This causes another problem that voltage is reduced unless electrode plates, an electrolyte and others are changed.
In light of the above problems, a lithium ion rechargeable cell has been contrived. This lithium ion rechargeable cell uses a wrap-type pouch case as a cell case, and contains an electrode group that is wrapped in a laminated film and depressurized so that the electrode group and the laminated film are firmly adhered together (Unexamined Japanese Patent Publication No. 2004-103415).
If the cell case is made with a wrap-type pouch case as described, it is possible to reduce the wall thickness of the cell case and smoothly pack the electrode group in the cell case without extra space. The lithium ion rechargeable cell can be configured without a change in outer size of the cell case. At least the module case fabricated with a rigid case is quite capable of absorbing external impacts.
On the other hand, if the electrode group is simply covered with a laminated film as described above, an adhesion degree between the laminated film and the electrode group is high. Due to the high adhesion degree, in spite that the lithium ion rechargeable cell needs to use a liquid electrolyte, the electrolyte cannot be sufficiently injected in between the laminated film and the electrode group.
If the electrolyte is not be sufficiently injected into the cell case, this raises problems including an increase in internal resistance, and also incurs a deterioration in power generation efficiency. The technology disclosed in the above-mentioned publication interposes an elastic body in between the laminated film and the electrode group. However, the situation is still the same.
One idea for solving this problem is to lower the adhesion degree between the laminated film and the electrode group. If the adhesion degree is lowered, the cell case becomes unstable in shape. This adversely affects the packageability of the cell case into the module case, that is, the manufacture efficiency of lithium ion rechargeable cells. Furthermore, if the adhesion degree is simply reduced, gaps between the laminated film and the electrode group would be uneven, and the electrolyte cannot be uniformly injected. This causes another problem that the power generation efficiency is not successfully improved.